1. Field of the Invention
The present invention relates generally to semiconductor image pickup devices and particularly to those which can pick up an image in a wide dynamic range if in a field of view there concurrently exist regions having a large difference in brightness, and which can also detect sufficient contrast in all regions.
2. Description of the Background Art
Charge coupled devices (CCDs), complementary metal-oxide semiconductors (CMOSs), imagers and other similar solid pickup devices, or so-called semiconductor image sensors (hereinafter also referred to as “semiconductor image pickup devices”) are incorporated in video cameras, digital cameras and the like. Such semiconductor image pickup devices are now also incorporated in mobile phones and the like and widely used as inexpensive and less power consuming pickup devices.
A semiconductor image pickup device, however, has a sensing ability significantly inferior to human visual perception. Human vision can sufficiently detect a contrast between a bright place and a dark place if in a single field of view there is a distribution in brightness of approximately four to five digits. This excellent ability to sense contrast is implemented because retina has therein photoreceptive cells, which can each adjust its receiving-light sensitivity individually.
In contrast, conventional semiconductor image pickup devices have pixels all having a uniform receiving-light sensitivity, and it is difficult to simultaneously obtain sufficient contrast at bright the dark spaces in a field of view.
FIGS. 29 and 30 show by way of example images picked up by a conventional semiconductor image pickup device. These images show a view which is shot in a sunny day inside a room with a fluorescent lamp turned on and also includes a view outside a window. Such a view is a common one often seen in daily life. It is apparent through experience that in such an ordinary view, the human eye can see the indoor view as well as that outside the window without difficulty in sufficient contrast.
Conventional semiconductor image pickup devices, however, have pixels all having identical receiving-light sensitivity and its dynamic range itself is significantly small. As such, if a user desires to pick up an image of a particular region in a single field of view, the other regions would be outside a region allowing an image to be picked up.
For example in the FIG. 29 example each pixel has its receiving-light sensitivity adjusted to be suitable for shooting an image of a darker region or an indoor view. This results in too bright an outdoor image, causing so-called whiteout, and the brighter portion's contrast cannot be detected.
In contrast, in the FIG. 30 example, each pixel has its receiving-light sensitivity adjusted to be suitable for shooting the brighter outdoor image. This results in too dark an indoor image, having so-called blackout, and the darker portion's contrast can hardly be detected.
Thus when there exist bright and dark regions simultaneously in a field of view, conventional image sensors have a significantly inferior capability of detecting sufficient contrast in all regions to human visual detection.
Furthermore, CDD/CMOS image technology: chapter 3 “Logarithmic transformation CMOS image sensor technology and application in Minolta/ROHM,” Kenji Takada et. al., Triceps, pp. 107-117 suggests a technique to address a limit in detecting whiteout, blackout and the like attributed to narrow dynamic range by a technique of a wide dynamic range camera employing a log amplifier and the like.
FIG. 31 shows by way of example an image picked up with a wide dynamic range camera employing a log amplifier as described above. A configuration employing a log amplifier does allow a wide dynamic range for each pixel, however a system is adapted to compress an image with the pixels all having a common receiving-light sensitivity. Less clear contrast is provided and the entirety of an image is blurred.
To address such a disadvantage, Japanese Patent Laying-Open No. 2000-340779 (herein after referred to as Patent Document 1) discloses that a wide photosensitivity range and a high contrast detection function are implemented by a configuration of a semiconductor image pickup device that can be driven by a quantity of light incident on an adjacent pixel to shift a photosensitivity range at each pixel circuit.
The configuration disclosed in Patent Document 1, however, requires that each pixel circuit be provided with a first photodetection element detecting a quantity of light received by the pixel circuit and a second photodetection element connected to that of another pixel circuit via a resistor to detect an average quantity of light received at adjacent pixel. This is an obstacle to providing a pixel having reduced size essential to meeting a recent demand for higher resolution.
Furthermore, as a single pixel circuit requires two photodetection elements therein, their quantities of light received, which should be equal, vary with their locations. More specifically, if the two photodetection elements are laterally arranged, incident light itself would not be uniform. If they are longitudinally arranged, then while incident light is uniform, wavelength dependence would be introduced, and the two photodetection elements hardly detect the same quantity of light received. This provides an inaccurate relationship between a quantity of light received and a voltage output and may provide impaired precision of detection.
Furthermore, the first and second photodetection elements are connected in each pixel circuit in series, a noise introduced into a node electrically connected to an adjacent pixel circuit may be superimposed on a photocurrent of the first photodetection element. This allows noise to be readily picked up, and impaired precision of detection may be provided.